1. Field of the Invention
The present application relates generally to the fabrication of secondary electrochemical cells, and more particularly, of nanostructured germanium-containing materials and alkali metal alloys thereof, all of which are useful as electrodes in secondary electrochemical cells.
2. Description of the Related Art
Batteries are used to power electrical devices that are not easily powered by a fixed power source, for example, portable electronics and spacecraft. Certain applications, for example, electric vehicles, are limited by the energy capacities of available rechargeable batteries, which are also referred to herein as secondary electrochemical cells.
Among the secondary electrochemical cells with the highest energy capacities are lithium batteries, which use lithium anodes. Commercially available lithium ion batteries typically use graphite-based anodes into which the lithium intercalates. The theoretical maximum stoichiometry of a graphitic anode is LiC6, which translates into a specific capacity of about 372 mAh/g. In certain cases, solvent cointercalation in the graphite anodes reduces the storage capacity from the theoretical value.
The energy density in a lithium ion battery may be increased by increasing the density of lithium in the anode, for example, by using a metallic lithium anode. Metallic lithium presents safety issues, however, which restrict metallic lithium anodes in secondary batteries to small cells. Moreover, cells with metallic lithium anodes tend to have limited lifetimes. Recharging a discharged or “dead” cell electroplates lithium onto the anode, which tends to grow as dendrites in a cell with a metallic lithium anode. In many cases, the lithium dendrites bridge between the anode and cathode of the cell, thereby creating an internal short circuit in the battery, which renders the battery unusable.
As noted above, lithium ion batteries typically use lithium-graphite anodes. The graphite acts as a framework material into which the lithium atoms can reversibly enter and exit, thereby controlling the electrode geometry or shape as the cell is recharged, and consequently, preventing dendritic growth. This framework material reduces the specific or gravimetric capacity of the electrode, however. Consequently, an ideal framework material has both low density and high lithium capacity.
An attractive framework material is silicon. Lithium-silicon alloys have low operating voltages versus lithium (˜300 mV), and large theoretical energy densities (up to 4200 mAh/g for Li4.4Si). A 300% volume increase accompanies fully lithiating silicon, however, resulting in mechanical stresses that pulverize the material within a few charge/discharge cycles. Moreover, slow lithium transport kinetics limit these anodes to medium and high temperature cells using molten electrolytes.
Nanostructured silicon and lithium-silicon alloys display improved room temperature cycle life compared to bulk-silicon electrodes. Examples of such materials are disclosed in copending U.S. patent application Ser. No. 10/660,382, filed on Sep. 10, 2003; S. Bourderau, T. Brousse, and D. M. Schleich, J. Power Sources, 81:233-236, 1999; H. Li, X. Huang, L. Chen, Z. Wu, and Y. Liang, Electrochem. Solid State Lett., 2:547-549, 1999; G. W. Zhou, H. Li, H. P. Sun, D. P. Yu, Y. Q. Wang, X. J. Huang, L. Q. Chen, and Z. Zhang, Appl. Phys. Lett., 75:2447-2449, 1999; J. Graetz, C. C. Ahn, R. Yazami, and B. Fultz, Electrochem. Solid State Lett., 6:A194-A197, 2003; and S. Ohara, J. Suzuki, K. Sekine, and T. Takamura, J. Power Sources, 119-121:591-596, 2003, the disclosures of which are incorporated by reference.
On exposure to oxygen, silicon forms a native oxide (silicon dioxide, SiO2) on its surface, which reduces the overall capacity of a silicon electrode, however. In the initial charging cycle for this type of electrode, the lithium reduces the silicon oxide, forming Li2O and elemental silicon, thereby resulting in a large irreversible capacity for the first cycle, as well as reducing the gravimetric capacity of the material. In a nanostructured material, the high surface to volume ratio means that native oxide accounts for a significant fraction of the silicon atoms in the material.